IMR, also known as high drain, are the recommended batteries for most variable voltage, or variable wattage, devices. Although the ICR batteries (protected) have a higher advertized mah rating, and may appear to be more attractive because of that, they are not designed for the short but high energy demands that boost circuits require to increase wattage or voltage. The IMRs will actually outperform the ICRs, hold a charge longer in use, and have a longer overall expected lifetime when both are used in variable voltage/wattage APVs.
I really don't recommend stacking batteries in a Silver Bullet, or any mechanical mod that has no electrical safety circuitry to help prevent a catastrophic battery event. The only safety features the Silver Bullet has is the hot spring, a ventable power switch, and a 3.5 amp limit on the switch. There is a good reason why Altsmoke strongly recommends against stacking batteries in the Silver Bullet. It is printed in bold red color on the Silver Bullet ordering page on their website. It is there for your safety.
If someone does choose to stack batteries, one must be diligent in permanently marking each battery (identifying) and using/charging them in pairs, and rotate those paired batteries in the order placed into the mod. The battery that ends up closest to the the positive connector (first battery in) becomes the most drained and if it happens to repeatively be the same battery in the pair they become unequal partners, resulting in a higher chance of a catastrophic event.
I personally don't understand taking that approach. By the time you buy enough of the RCR123 batteries (they don't hold a charge for very long used in this fashion), a charger, and a voltmeter to check them after usage and after coming off the charger, you could have bought a Kick for a much safer entry into higher voltage.